Acute Kidney Injury

Acute kidney injury is an abrupt decline in kidney function, identified by a rise in serum creatinine or a fall in urine output over hours to days. It is common in hospitalized and critically ill patients, is conventionally divided into prerenal, intrinsic, and postrenal causes, and ranges from readily reversible functional impairment to established tubular injury.

Definition

Acute kidney injury is a rapid (hours to days) reduction in glomerular filtration, operationally defined by an increase in serum creatinine and/or a decrease in urine output, and graded by severity using consensus criteria.

Scope

This entry covers acute kidney injury as a pathophysiologic and diagnostic category: its definition and staging, the prerenal/intrinsic/postrenal framework, the pathology of acute tubular injury, and the link between acute injury and later chronic kidney disease. It describes how the condition is defined and studied rather than offering management guidance.

Core questions

• Is the cause prerenal (hypoperfusion), intrinsic (parenchymal injury), or postrenal (obstruction)?
• How are the definition and staging of acute kidney injury standardized across studies?
• What distinguishes reversible functional impairment from established acute tubular injury?
• Why does an episode of acute kidney injury raise the long-term risk of chronic kidney disease?

Key concepts

• Prerenal, intrinsic, and postrenal causes
• Serum creatinine and urine output criteria
• KDIGO staging of acute kidney injury
• Acute tubular injury (acute tubular necrosis)
• Ischemia and nephrotoxic injury
• Oliguria
• Transition from acute injury to chronic kidney disease

Mechanisms

Acute kidney injury is conventionally grouped by mechanism. Prerenal injury results from reduced renal perfusion (hypovolemia, low cardiac output, or altered intrarenal hemodynamics) and is initially a functional, reversible state. Intrinsic injury involves the kidney parenchyma; the most common form is acute tubular injury from ischemia or nephrotoxins, in which tubular epithelial cells are damaged, slough into the lumen, and impair filtration through tubular obstruction and back-leak. Postrenal injury reflects obstruction to urine flow. Sustained or severe injury, and incomplete tubular repair, can leave residual fibrosis that links an acute episode to subsequent chronic kidney disease.

Clinical relevance

Acute kidney injury is common among hospitalized and critically ill patients and is associated with increased morbidity and mortality and with progression to chronic kidney disease. This entry explains how acute kidney injury is defined, classified, and studied and is not a source of diagnostic or treatment recommendations for individual patients.

Epidemiology

Acute kidney injury occurs in a substantial fraction of hospitalized patients and is especially frequent in intensive care, where it carries high mortality. Systematic review evidence links episodes of acute kidney injury to an increased subsequent risk of chronic kidney disease and kidney failure.

History

The condition was long described as acute renal failure, with wartime crush-injury cases helping to define ischemic and nephrotoxic tubular injury. The terminology shifted toward acute kidney injury to capture a continuum from mild functional change to severe failure, and consensus definitions — RIFLE, AKIN, and the KDIGO criteria — standardized its diagnosis and staging.

Debates

How completely does acute kidney injury reverse?

Acute kidney injury was once viewed as largely reversible, but accumulating evidence that episodes predispose to later chronic kidney disease has reframed it as a potential driver of long-term kidney damage rather than a purely transient event.

Seminal works

• bellomo-2012
• khwaja-2012
• coca-2012

Frequently asked questions

How is acute kidney injury defined?

It is defined by an acute rise in serum creatinine and/or a fall in urine output over hours to days, graded by severity using consensus criteria such as those of KDIGO.

What is the most common intrinsic cause of acute kidney injury?

Acute tubular injury (also called acute tubular necrosis), most often from ischemia or nephrotoxic exposure, is the most common intrinsic parenchymal cause.